1. Field of the Invention
This invention relates to a printing apparatus for depositing a liquid composition on a surface, such as the depositing of a liquid composition containing an organic semiconductor material on an backplane, and particularly to a system and corresponding method for replenishing the supply of the liquid composition.
2. Description of the Related Art
Organic electronic devices utilizing organic active materials are used in many different kinds of electronic equipment. The term “organic electronic device” is intended to mean a device, such as an organic light emitting diode (OLED), that includes one or more layers of organic semiconductor materials laminated between other supporting layers and sandwiched by two electrodes.
Each organic material is carried in a liquid composition. During manufacture of a device each liquid composition is dispensed from a dedicated nozzle carried by a dispensing bar. The nozzles are grouped in nozzle sets, with one nozzle in each set dispensing a particular color of ink. Each nozzle dispenses liquid and deposits that liquid along a longitudinal lane that extends across a backplane of the device. The nozzles in each set continuously dispense a liquid composition into a respective lane as the bar traverses the backplane.
The individual nozzles for each particular color in each nozzle set are supplied as a group from a common manifold itself supplied from a suitable nozzle supply source, or supply reservoir. The supply reservoir for each particular color is usually implemented as a communal reservoir. The supply reservoir may either directly hold a supply of liquid for the nozzles, or may hold a secondary container, such as a sealed pouch containing the particular colored liquid composition.
The supply reservoir used in the prior art includes an outer shell, or can, that is closed in an air-tight manner by a conjoinable cover. When conjoined the can and cover cooperate to define an enclosed interior chamber. The interior chamber of the supply reservoir contains an inert gas (e.g., nitrogen) that is held at a predetermined pressure above atmospheric pressure, typically a pressure level on the order of one hundred thirty pounds/square inch (130 psi; 0.9 MPa). The pressure of the inert gas in the interior chamber forces liquid from the supply reservoir or from the secondary container to the manifold, and from the manifold, through a dedicated flow line to a nozzle.
The holding capacity of the supply reservoir (or the secondary container) is limited, requiring that the ink supply in the supply reservoir be replenished as the liquid is consumed by the nozzles. Several factors serve to complicate ink replenishment.
Currently, replenishing the supply reservoir requires a shut-down of the printer and termination of nozzle discharge. The dispensing vessel is depressurized and its lid removed. With the resevoir open the secondary container must be either replaced or refilled. Once the supply reservoir is replenished with liquid the lid is re-attached and the reservoir is re-pressurized.
This arrangement and method for replenishing liquid is believed disadvantageous for several reasons.
On a system level care must be exercised to minimize the potential of nozzle failure due to plugging, which is more probable to occur during startup/shutdown phases than during steady operation of the system. The more often nozzles are turned off the greater is the chance for nozzle failure. Moreover, after starting a nozzle it takes a significant amount of time for the system to reach a steady flow rate. During this time the ink discharged from the nozzle is not useable and is wasted.
The act of replacing or refilling of the pouch (if a pouch is used as the secondary vessel) involves additional specific challenges beyond the disadvantages caused by the time-consuming disconnection and re-connection of the pouch to its associated manifold. These challenges include maintaining the pouch's structural integrity, avoiding gas entrapment in any supply lines, avoiding the introduction of atmospheric gases into the pouch, and monitoring the volume of liquid introduced into the back to prevent overfill and potential rupture of the pouch.
U.S. Pat. No. 5,473,350 (Mader et al.) discloses a system and method for maintaining ink concentration in a printing system. However, in the system disclosed in this patent it appears that the main ink supply reservoir is maintained under partial vacuum, rather than at a pressure greater than atmospheric pressure.
In view of the foregoing it is believed advantageous to provide an apparatus and method for continuously replenishing a liquid composition in the reservoir serving as the nozzle supply source without requiring disconnection or de-pressurization.